If we're to believe some of the latest scientific research on sleep (or even just viral YouTube videos), the effects of dreams have likely bewildered Earth's creatures for hundreds of millions of years. But humankind, in particular, has fostered a unique and lasting fascination with dreaming.

Yet for all our interest, there remains much about dreams and their underpinnings that we simply don't understand — and we're learning a lot more all the time. Here are ten things you probably didn't know about dreams.

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10) There is a science dedicated to the study of dreams
First things first, let's get one thing straight. When many people think about the study of dreams, what they're actually thinking of is the practice of dream interpretation; but interpretation is very different from the scientific study of dreams, known as Oneirology.

The difference? Oneioroloists aren't necessarily concerned with the meaning of dreams, so much as they are with the mechanisms and processes that give rise to them.

9) Dream Interpretation has been around for a long, long time
A lot of people associate dream interpretation with modern psychological analysis, but by the time the likes of Jung and Freud got around to it, the practice of dream interpretation had been in full swing for thousands of years.

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Some of the first evidence of dream interpretation dates all the way back to the to the 3rd millennium BC, to the ancient cultures of the Mesopotamian. These early civilizations were not only among the first to develop writing, they also practiced dream interpretation regularly, collecting the accounts of dreams (especially those of royal figures) into dream books, complete with interpretations.

8) Everyone tends to dream about the same thingsIn a study conducted in 2004, scientists from the Sleep Laboratory at the Central Institute of Mental Health in Mannheim, Germany administered a "Typical Dream Questionnaire" to 444 participants in an effort to characterize the variability of dream content across their test population. The test subjects were asked to identify how many of 55 "typical dream themes" (like being chased, having your teeth fall out, flying, running in place, etc) they had experienced. According to the researchers:

The findings indicated that most of the 55 dream themes occurred at least once in most of the participants' lifetimes. In addition, the correlation coefficients for the rank order of the themes were very high; that is, the relative frequencies were stable.

Sexual intercourse was the most common type of sexual content, followed by sexual propositions, kissing, and fantasies... masturbation accounted for approximately 6% of both male and female sexual dreams and an orgasm was experienced in approximately 4% of all sexual dreams.

6) Dreams can be a sad, scary place
The findings made by the German scientists in number 8 built upon those of many others, most notably psychologist Calvin Hall's. Over the course of several decades, Hall collected over 50,000 dream reports, and found that the vast majority of them contained similar thematic elements. They were so similar, in fact, that he and his colleague, fellow psychologist Robert Van de Castle, developed a system of dream classification called "the Hall/Van De Castle system of dream content analysis." According to the UC Santa Cruz website on dream research:

Since its publication, the Hall/Van De Castle system of dream content analysis has been used by many different investigators in the United States, Canada, Europe, India, and Japan. Hall himself applied it to dream reports collected for him in four Latin American countries and by anthropologists in many different preliterate societies. All of these studies, incidentally, showed there was more aggression than friendliness, more misfortune than good fortune, and more negative emotion than positive emotion in dream reports from all around the world; when these dream reports were compared to those from industrialized nations, the similarities far outweighed the differences.

5) Not everyone dreams in color
While it's believed that the majority of us dream in color, its estimated that roughly one person in eight is limited to black and white dreamscapes. But this wasn't always the case. Research on dreams from the first half of the 20th century suggests that the vast majority of people actually used to dream in black and white. But beginning in the sixties, the balance began tipping in the direction of color dreaming. What accounted for this shift? According to Dundee University's Eva Murzyn, the advent of Technicolor (i.e., color movies and television):

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"It suggests there could be a critical period in our childhood when watching films has a big impact on the way dreams are formed."

But here's the real kicker: according to Murzyn, if one looks even further back in history — back before even black and white television came on the scene — all evidence suggests we were dreaming in color.

4) People trying to kick a smoking habit tend to experience more vivid dreams
Regular smokers who suddenly kick the habit are likely to experience a number of pretty rough withdrawal symptoms, but one that you don't hear about very often is the effect that quitting has on a person's tendency to dream. A study published in the Journal of Abnormal Psychology reports:

Among 293 smokers abstinent for between 1 and 4 weeks, 33% reported having at least 1 dream about smoking. In most dreams, subjects caught themselves smoking and felt strong negative emotions, such as panic and guilt. Dreams about smoking were the result of tobacco withdrawal, as 97% of subjects did not have them while smoking, and their occurrence was significantly related to the duration of abstinence. They were rated as more vivid than the usual dreams and were as common as most major tobacco withdrawal symptoms.

3) Your motor neurons cease to be stimulated during REM sleep, leaving you paralyzed
Speaking of rapid-eye-movement sleep, when you're in a state of REM sleep, your body's release of neurotransmitters like norepinephrine, serotonin, and histamine—all of which play an important role in stimulating motor neurons—is completely suppressed.

The result is a condition known as REM atonia, wherein your muscles enter a state of relaxation that borders on physical paralysis. It is thought that this loss of mobility helps keep you from reacting to your dreams in ways that might result in physical harm. In fact, people who don't experience REM atonia may suffer from what is known as REM behavior disorder, and unconsciously act out their dreams in a way that results in injury to themselves or others.

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2) Several other species experience complex brain activity during sleep
Many of the patterns of brain and physiological activity that humans experience during sleep—including REM sleep and its associated brain states—have been observed in a number of animals, including other mammals, birds, and reptiles.

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1) Our understanding of dreams remains very limited
Consider how much we've already talked about REM-sleep. And yet, for all the research that's been done on REM sleep and its role in a person (or animal's) dream state, our understanding of sleep's deeper mechanisms remain muddled, and there still exists no clear biological definition of the phenomenon.

Consider, for example, that the link between REM and dreaming was only made as recently as the 20th Century (read the first paper to describe this connection, published in a 1953 issue of Science, here). For decades it was assumed that REM sleep was necessary for dreams to occur. It took until 2001, and the publication of this study in the Journal of Sleep Research for us to prove that REM sleep is, in fact, not necessary for dreams to occur.